The solids deposited on the inner surface of rotary kilns are usually scraped off by periodically reciprocating a boring bar during the operation of the kiln. However, it is difficult to bring the cutting edge on the front end of the boring bar close to the wall of the kiln, because the boring bar merely travels reciprocally along the axis of the kiln. Especially with kilns which are provided with a tuyere dam of specified height at the outlet of the kiln to retain the feed to be treated, the boring bar can be brought close the kiln wall only to such an extent that the bar will not engage the tuyere dam, consequently permitting part of the solid deposits unremoved.
Since rotary kilns are equipped at the outlet with a burner for injecting a high-temperature gas, solids tend to lodge mainly on the outlet portion of the kiln. Accordingly, it has been highly desired to provide an apparatus for removing solid deposits from the kiln wall beyond the tuyere dam at the outlet of the kiln.